The extruded polystyrene foam is continuously produced by heating and melting a styrene resin composition using an extruder or the like, adding a foaming agent under high-pressure conditions, cooling the resultant mixture to a predetermined resin temperature, and then extruding the resultant mixture into a low-pressure area.
The extruded polystyrene foam is used as, for example, heat insulating materials for structures due to good workability and insulation properties. In recent years, a demand for energy saving of residences, buildings, and the like has increased, so that a technical development of a higher heat insulating foam than before has been desired.
Heretofore, chlorofluorocarbon (hereinafter referred to as CFC), such as dichlorodifluoromethane, has been widely used as a physical foaming agent for use in production of the extruded polystyrene foam. However, CFC has a high risk of destroying the ozone layer, and therefore a hydrogen atom containing chlorofluorocarbon (hereinafter referred to as HCFC) with a small ozone depletion potential has been used as a substituent for the CFC. However, the ozone depletion potential of the HCFC is also not 0 (zero), and therefore it cannot be said that the HCFC is completely free from a risk of destroying the ozone layer. Thus, in recent years, hydrofluorocarbon (hereinafter referred to as HFC) which has an ozone depletion potential of 0 (zero) and does not have a chlorine atom in the molecules has been increasingly used as the foaming agent.
For example, Patent Document 1 discloses, as a method for producing a polystyrene resin foam which has excellent heat insulation performance over a long period of time and which can be suitably used for heat insulating materials for houses and the like using a front-based foaming agent having an ozone depletion potential of 0, a method for producing a foam having a density of 2×10−2 to 4.5×10−2 g/cm3 including pressing a foaming agent obtained by mixing trifluoroethane which is one kind of HFC and methyl chloride into a polystyrene resin for extrusion foaming. However, there has been a problem that the HFC has a high global warming potential.
Thus, a method for producing an extruded polystyrene foam insulating board using a fluorinated olefin (hydrofluoroolefin, which is also referred to as HFO) which has an ozone depletion potential of 0 (zero) and has a small global warming potential, and thus is friendly to the environment as an alternative foaming agent of the HFC has been proposed (for example, refer to Patent Documents 2 to 5). However, these former techniques have not succeeded in obtaining an extruded polystyrene foam having excellent heat insulation properties and flame retardancy sufficiently utilizing the merits (low thermal conductivity, flame retardancy) of using the hydrofluoroolefin, and thus these former techniques still have had problems.